Clustered Regularly interspaced Short Palindromic Repeats (CRISPR), in combination with associated sequences (cas) constitute the CRISPR-Cas system, which confers adaptive immunity in many bacteria. CRISPR-mediated immunization occurs through the uptake of DNA from invasive genetic elements such as plasmids and phages, as novel “spacers.”
CRISPR-Cas systems consist of arrays of short DNA repeats interspaced by hypervariable sequences, flanked by cas genes, that provide adaptive immunity against invasive genetic elements such as phage and plasmids, through sequence-specific targeting and interference (Barrangou et al. 2007. Science. 315:1709-1712; Brouns et al. 2008. Science 321:960-4; Horvath and Barrangou. 2010. Science. 327:167-70; Marraffini and Sontheimer. 2008. Science. 322:1843-1845; Bhaya et al. 2011. Anna. Rev. Genet. 45:273-297; Terns and Terns. 2011. Curr. Opin. Microbiol. 14:321-327; Westra et al. 2012. Anna. Rev. Genet. 46:311-339; Barrangou R. 2013. RNA. 4:267-278). Typically, invasive DNA sequences are acquired as novel “spacers” (Barrangou et al. 2007. Science. 315:1709-1712), each paired with a CRISPR repeat and inserted as a novel repeat-spacer unit in the CRISPR locus. Subsequently, the repeat-spacer array is transcribed as a long pre-CRISPR RNA (pre-crRNA) (Brouns et al. 2008. Science 321:960-4), which is processed into small interfering CRISPR RNAs (crRNAs) that drive sequence-specific recognition. Specifically, crRNAs guide nucleases towards complementary targets for sequence-specific nucleic acid cleavage mediated by Cas endonucleases (Garneau et al. 2010. Nature. 468:67-71; Haurwitz et al. 2010. Science. 329:1355-1358; Sapranauskas et al. 2011. Nucleic Acid Res. 39:9275-9282; Jinek et al. 2012. Science. 337:816-821; Gasiunas et al. 2012. Proc. Natl. Acad. Sci. 109:E2579-E2586; Magadan et al. 2012. PLoS One. 7:e40913; Karvelis et al. 2013. RNA Biol. 10:841-851). These widespread systems occur in nearly half of bacteria (˜46%) and the large majority of archaea (˜90%). They are classified into three main CRISPR-Cas systems types (Makarova et al. 2011. Nature Rev. Microbiol. 9:467-477; Makarova et al. 2013. Nucleic Acid Res. 41:4360-4377) based on the cas gene content, organization and variation in the biochemical processes that drive crRNA biogenesis, and Cas protein complexes that mediate target recognition and cleavage. In types I and III, the specialized Cas endonucleases process the pre-crRNAs, which then assemble into a large multi-Cas protein complex capable of recognizing and cleaving nucleic acids complementary to the crRNA. A different process is involved in Type II CRISPR-Cas systems. Here, the pre-CRNAs are processed by a mechanism in which a trans-activating crRNA (tracrRNA) hybridizes to repeat regions of the crRNA. The hybridized crRNA-tracrRNA are cleaved by RNase III and following a second event that removes the 5′ end of each spacer, mature crRNAs are produced that remain associated with the both the tracrRNA and Cas9. The mature complex then locates a target dsDNA sequence (‘protospacer’ sequence) that is complementary to the spacer sequence in the complex and cuts both strands. Target recognition and cleavage by the complex in the type II system not only requires a sequence that is complementary between the spacer sequence on the crRNA-tracrRNA complex and the target ‘protospacer’ sequence but also requires a protospacer adjacent motif (PAM) sequence located at the 3′ end of the protospacer sequence. The exact PAM sequence that is required can vary between different type II systems.
The present disclosure provides methods and compositions for increasing the efficiency and specificity of synthetic type II CRISPR-Cas systems in such uses as, for example, site-specific cleavage, site-specific nicking, transcriptional control and genome editing.